1. Field of the Invention
The present invention relates to an apparatus enabling the collection and recognition of electrical signals that are mixed with other signals that impede direct access; these other signals may be either intentional parasitic signals (hostile jamming) or unintentional ones (under transmission conditions), or encoding signals for protection against recognition of characteristic signals by an unauthorized person.
2. Description of the Prior Art
Any noncharacteristic signals that impede the detection and recognition of signals that comprise messages, such as data transmission, orders or other significant elements, are generally classified as "noise".
One application of the method according to the invention relates to radionavigation and position-finding systems that include a receiver for signals transmitted by a set of transmitter beacons. The beacons are either located on Earth (on the ground or at sea) or on satellites. Periodically and synchronously, they emit encoded signals containing the three exact spatial and temporal coordinates of the transmission. By cross-referencing the propagation delays and the spatial coordinates of signals transmitted by several beacons, the navigation or position-finding system can calculate the position of its receiver.
There are other applications of the invention as well, such as in the telecommunications field, especially for decoding purposes.
As is well known per se, significant signals are superimposed on a fixed-frequency transmission, known as a "carrier frequency", or more simply, "carrier".
In the prior art, the signals are collected by a collector means such as an antenna, a coaxial cable, an electro-optical cable or the like, depending on the chosen transmission system.
The collector is associated with a receiver, which performs the demodulation of the signals received, to separate the carrier frequency from the significant signals that are to be decoded.
After this conventional preprocessing, electrical signals, comprising the wave to be recognized and noise, are then processed.
The wave to be recognized is generally a succession of fixed-frequency bits, a bit being defined as either a voltage level, that is, +V or -V, or the alternation of two voltages: from -V to +V (generally from -5 V to +5 V) or from +V to -V.
With a view to the recognition of desired signals (trains of elementary signals) on the desired carrier, one known method comprises performing what is known in mathematics as a convolution; it involves generating a train of signals similar to the train to be recognized, and finding the product of the two trains, which a priori are out of phase. The summation of this product over the length of the test train comprises the outcome of the convolution. The product of convolution is a function of the displacement between the test train and the train to be recognized.
It is known that this product is maximal when both trains are in phase, that is, when their displacement is zero.
A peak value of the product of convolution thus means that there is perfect coincidence between the test train and the train received, i.e., it is recognized that the received train is indeed the train that was sought.
These operations are generally performed electronically. The test train and the received train are compared bit by bit (elementary signal by elementary signal); that is, to compose the test train, a first bit that is identical to the first bit of the train to be recognized is generated; then this first bit is put in phase with the first bit of the received train; then a second bit, identical to the second bit of the train to be recognized, is generated; then this second bit is put in phase with the second bit of the train to be recognized; and so forth.
As soon as any bit generated for the test train is not in phase with the corresponding bit of the received train, the first bit generated is put in phase again with a new received bit, and the process starts over, until each of the bits of the test train are exactly in phase with each of the corresponding bits of the received train.
This method is very time-consuming, and despite the improvement it provides in the operating speed of the associated processing units (computers), it slows down the recognition of the received trains substantially. For example, when this method is used in a radionavigation system, it delays the determination of the position of the receiver, and consequently it yields erroneous information, since by the time the position is found, the moving body that carries the receiver will have traveled onward a certain distance, and will then be at a different position from the one indicated.
The uncertainty engendered by the slowness of this method may be of small importance for certain slow-moving bodies; but for very fast-moving bodies, such as missiles, satellites, supersonic aircraft, and the like, it becomes critical.
Currently, the highest-powered systems include a Bragg cell, which is an acousto-optical transducer essentially comprising a crystalline body that receives both radiation and the received electronic signals; these signals influence the returned radiation.
The Bragg cell is thus associated with the receiver and modulates a beam of light as a function of the signals received. Means are provided for generating a train of signals corresponding to a train to be recognized, and for introducing it into the Bragg cell in a direction substantially parallel to the direction of propagation of the signals received. At least one optoelectronic detector is disposed downstream of the Bragg cell (in terms of the direction of propagation of the beams in the crystalline body), and a processing unit associated with the optoelectronic detector determines the variations in modulation of the beams, either to deduce the instant of synchronization of the received train and the test train, or to slide the test train relative to the trains received until synchronization is obtained, which by definition occurs if and when the optoelectronic detector receives a maximum light intensity, which corresponds to the maximum of the convolution function.
With a system of this type, the time needed to synchronize the test train and the train to be recognized depends essentially on the product of the period of the train to be recognized multiplied by the number of bits in a train, because the product of convolution must be found as many times as there are bits in a train, and the duration of a product of convolution is the period of the train to be recognized.